Patent GB-539501-A (GB)
Title: Improvements in and relating to electrical storage and delay circuits
IPC: H03H7/32
Filed: 1940-04-08
Abstract: 539,501. Cathode ray oscillographs; frequency measurements. HADFIELD, B. M. April 8, 1940, No. 6290. [Class 37] A circuit for time-delaying a sequence of voltage-time functions comprises one or more branch circuits each including a rectifier and a condenser in series therewith, the condenser being charged by means of the rectifier in dependence upon the voltage-time function, and means controlled by the voltage-time function for discharging the condenser to pass on the charge to subsequent equipment, the arrangement being such that the discharging of the condenser is completed at or before the time of arrival of the next voltage-time function at that condenser. The circuit is applicable to target diagram apparatus as described in Specification 502,143, in which voltages proportional to the logarithm of the make-andbreak periods of an impulsing contact are applied to the deflecting plates of a cathode ray tube, the application of the voltages being delayed until both the make-and-break periods of one impulse have elapsed. As shown in Fig. 1, a branch circuit consists of a rectifier MR in series with a condenser C1 and a contact S2 for discharging the condenser through a resistance R1. It is assumed that the input voltage-time function has been established on a condenser C and that a contact S1 controlled by the voltage-time function switches the condenser on to a resistance R. The voltage wave-form on R is of the impulsive type and is applied to the branch circuit. The contact S2 is open at the moment S1 closes, so that a voltage is established on C1 through the rectifier MR and remains there for a time dependent on the time constant of C1 and the back resistance of the rectifier. The circuit thereby effects storage and delay by a time equal to that of the original voltage-time function or pulse. When the next voltage-time function or pulse occurs S, is opened and S2 closed. The voltage on C1 then discharges impulsively through R, and can be applied to a further branch circuit to be stored again and delayed by the time interval between the first and second pulses, and so on. Figs. 2 and 3 show the application to the target diagram apparatus described in Specification 502,143, so as to eliminate the stepping switches. In the make period circuit, Fig. 2, a high-speed relay repeats the impulses to be tested and has a change-over contact X1, of which the moving contact is connected to the positive pole of a battery, and the break contact (normally made) to the junction of a charging resistance Rx and the condenser Cx, the other end of the resistance Rx being connected to the negative pole of the battery and the other side of the condenser Cx being connected through a discharge resistance R2 to the positive pole of the battery. For the duration of the make period the break contact of X1 is broken and the condenser Cx charges via Rx, R to a voltage representing the logarithm of the make period. When the break contact restores to normal the condenser Cx discharges in a much shorter time than the charging time. The voltage wave-form of the discharge is of the impulsive type and is applied to the branch circuit consisting of a rectifier MR1 and condenser C2, the junction of which is connected through a discharge resistance P to the make contact of X1. The condenser C2 remains charged to a voltage proportional to the initial voltage on Cx at the moment of discharge, for a time equivalent to the break period of X1. The next make period dissipates this voltage through the resistance P and the make contact. The break period circuit comprises a charge resistance Ry, Fig. 3, charge condenser Cy, discharge resistance R3, rectifier MR2, and a storage condenser C3, and is operated by a high-speed relay having a contact Y1. The make contact of Y1 is connected to the junction of Cy and Ry so that the charge on Cy corresponds to the break period, whilst the break contact is connected through a discharge resistance R4 to the junction of the condenser C3 and the rectifier MR2. The voltage on C3 persists for the make period of Y1 and is proportional to the logarithm of the break period, and is available after the first break period. The voltage representing the break period must be delayed until the end of the following make period before simultaneous use can be made of the break and make voltages. The voltage on C 3 is therefore delayed by another branch circuit so that it is available after the first make period. A further branch circuit is connected to the resistance R4 to pass the impulsive discharge voltage of the condenser C3 to a further condenser C4 through another rectifier MR3 at the moment when Y1 restores to normal, i.e. after the first make period. The voltage on the condenser C4 is dissipated at the commencement of the next make period by a further make contact B1 and resistance Q, the change-over contact of B1 being connected to the break contact of Y1. Steady voltages are simultaneously available on the condensers C2, C4 for the duration of the break period, of values proportional to the logarithms of the make and break periods of the impulses, and are applied to the two pairs of deflection plates of a cathode ray tube. The frequency of operation of the circuit elements may be increased by replacing the contacts by switching devices such as thermionic or gas-filled valves, controlled by the impulsive voltage inputs. Fig. 4 shows a circuit for use with an impulsive voltage-time function in which a positive halfwave is followed by a complementary negative half-wave. Four storage and delay circuits I, II, III, IV are employed and include four thermionic valves V1, V2, V3, V4 which are alternately switched by the negative and positive wave-fronts applied to the input terminals 1, 2.. When the terminal 1 is positive with respect to the terminal 2, the condenser C5 is charged. The negative potential on terminal 2 makes the grids of the valves V1,V3 negative so that these valves do not conduct and prevent the condensers C5, C7 from discharging. When the terminal 2 is positive with respect to terminal 1 the valves V1, V3 conduct, so that the condenser C5 discharges through the valve V1 and resistance R1 to cause the charging of the condenser C6. This charge is maintained owing to the rectifier MR5 and to the fact that the valves V2, V4 do 'not conduct, owing to negative potential on the grids. When the next voltage-time function arrives at terminals 1, 2, terminal 1 is again positive so that the condenser C6 discharges and condensers C5, C7 are charged. In this manner successive voltage-time functions are transferred successively from one circuit to the next. The number of circuits may be added to, whilst the output may be taken between terminals 3, 4 or 4, 5 to give an impulsive or steady output respectively. The input impedance to the first branch circuit may be formed by the cathode resistance of a valve of the cathode follower type having a high resistance in the grid lead. Fig. 7 shows a circuit for obtaining a target diagram from an impulsing contact T1, which is connected via a battery B 3 to the input resistance R 3 3 of a valve V6 so that closure of T1 increases the anode current, grid current being limited by a resistance R32. The make period charge condenser Cx discharges from the previous anode voltage to the lower voltage on V. through resistances R2, Rx, R23 and the valve V6, the rectifier MR12 being non- conducting. At the termination of the make period V6 becomes a very high resistance, so that Cx charges from the busbar voltage through resistances R2 2, R2, and the forward resistance of the rectifier MR12 within about 2 milliseconds. The resultant voltage pulse on R 2 is positive to the grid of the valve V6 and of magnitude corresponding to the logarithm of the make period. The valve V7 acts as a cathode follower so that the voltage on R 2 is in phase with and of the same magnitude as the input grid pulse. The voltage pulse on R25 is applied to the rectifier MR1, condenser C 2, and discharge valve V8; so that the make period voltages are available between terminals 6, 7. The switching of the valve V8 at the beginning of the next period is obtained by applying the impulsive voltage change on the anode resistance R24 of the valve V7, due to a negative pulse applied to the grid across Rx, R2 during the discharge of Cx, to the grid of V8 through a condenser C12 and resistance R19. The condenser C2 is then discharged in the minimum pulse time of 10 mini-seconds. The circuit for the break period includes two branch circuits. and the discharge valve V11 includes a resistance R18 to pass the voltage on to the second element. Discharge of the condenser C4 is effected by applying a positive pulse to the grid of the valve V12 from the resistance R29, since at this moment the valve V11 is non- conducting. The input charge and discharge valve V9, corresponding to V6, is operated therefrom by a high resistance potentiometer R34, R35. The circuit shown in Fig. 7 measures the half-cycle periods of the input wave-form and therefore may be used as a frequency meter by causing the valve V6 to act as a peak. chopper. By using a cathode ray tube as the indicator the instantaneous' frequency of the input may be read. The frequency spectrum of speech or music may thus be obtained. The remaining co-ordinate axis may be used for the peak value or time duration of the input.